Many petroleum reservoirs in the earth, when tapped by conventional oil and gas-well drilling techniques, yield "sour" oil and gas. The oil and gas are termed "sour" because of the presence therein of numerous sulfur-based compounds such as hydrogen sulfide and sulfur dioxide.
To be commercially useful, the oil and/or gas must be "sweetened" which requires removal of the sulfur-based substances from the oil and gas. The sulfur-based compounds are usually reduced to elemental sulfur in order to provide a marketable commodity. When sulfur markets are soft, the excess quantities of sulfur extracted from sour natural gas and oil are typically melted and stored in the field or in the immediate vicinity of the gas or oil production plant by casting the sulfur layer by layer in the same manner as concrete into large blocks. In Alberta, for example, many sulfur blocks cast in this way may measure one to three hundred yards in length, one to two hundred yards in width and ten to fifteen yards in height. It is estimated that in Alberta alone, there are about 18 million tons of sulfur cast in block form in the field.
Storage as solid blocks is advisable because storage in powder form can be an environmental and safety hazard. Sulfur dust, when blown by the wind, can cause environmental damage to surrounding property by creating an acidic environment which contributes to soil breakdown, which is detrimental to farming and ranching. Sulfur dust is also potentially explosive and a fire hazard.
Sulfur has important commercial value as a starting material in a wide range of chemical processes and as a fertilizer. However, since transportation to various markets of sulfur in large block form is uneconomical, it is necessary to reduce such blocks into tractable portions. Simple mechanical means for the breaking up of sulfur blocks in situ, for example, crushing devices, cutting means such as saws, or even explosives, have been employed in the past. But all such methods have been found to generate a large amount of undesirable sulfur dust. Furthermore, mechanical break-up of block sulfur is a relatively expensive procedure and yields pieces of irregular size, thereby detracting from handling efficiency and raising costs.
In recent years, in response to increasingly stringent environmental anti-pollution standards imposed by applicable regulatory bodies, apparatus and methods have been developed for melting sulfur from the large blocks of stored sulfur. The method usually involves contacting the sulfur block with a solid hot element heated electrically, or by internally circulated heating fluid. The melted sulfur obtained can then be transported, either in liquid form, or as re-cast uniform solid pellets. This type of apparatus is usually described as a "sulfur remelter". Examples of such apparatus are illustrated in U.S. Pat. No. 4,050,740, issued Sept. 27, 1977, Ellithorpe, U.S. Pat. No. 4,203,625, issued May 20, 1980, Ellithorpe, Canadian Pat. No. 1,040,037, issued Oct. 10, 1978, Bowman and Canadian Pat. No. 1,070,928, issued Feb. 5, 1980, Potts et al.
The sulfur remelters disclosed in these patents employ a sulfur melting technique which involves positioning a heating element in contact with a sulfur block and moving the element into the block as melted sulfur is removed, either by gravitational flow or by suction. A serious disadvantage of such melting devices and the overall technique is that the melting action is slowed or stopped by impurities or obstructions in the solid sulfur block. These impurities may be items such as steel forming spikes, wood planks, shovels, or other solid objects which were used when the sulfur was cast. Stones, pebbles, sand and dirt are other common impurities. In certain instances, it is necessary to withdraw the remelter from the block face, and remove the obstruction before remelting can be continued.
Another disadvantage is that existing sulfur remelting methods are incapable of melting sulfur blocks to ground level, or cannot deal with sulfur blocks cast on uneven ground. Pads or ground deposits of solid sulfur of one foot or more in thickness remain after using the conventional remelters as described, even on level ground. Also, where deposits are on uneven ground, for example, the ground may be fifteen or more feet higher at one end than the other end, the pads left in place may be generally wedge-shaped. It is necessary in such situations to melt the blocks in steps which thus leaves a stepped pad on the ground. Leaving sulfur pads in place after remelting the bulk of the sulfur is unacceptable both from an economic standpoint and an environmental standpoint. It is estimated that about 2 million tons of sulfur will remain as pads after conventional sulfur remelters have removed the upper portions of the existing stored sulfur blocks in Alberta.
The melting of block sulfur by direct contact with a solid heating element also presents a number of practical difficulties which arise from the peculiar physical-chemical properties of elemental sulfur. Solid sulfur raised from an ambient temperature begins to melt over a narrow temperature interval centred about 240.degree. F. (115.degree. C. ). Above that point, as with most liquids, the viscosity of the liquid sulfur decreases with increasing temperature until a minimum liquid sulfur viscosity is attained at about 310.degree. F. (155.degree. C.). Above about 310.degree. F. (155.degree. C.), however, the liquid sulfur undergoes a phase transformation over a relatively small temperature range which causes the viscosity of the liquid sulfur to increase sharply such that the sulfur becomes syrupy and sticky. The viscosity values for liquid sulfur at 250.degree. F. (120.degree. C.), 300.degree. F. (150.degree. C.), 330.degree. F. (165.degree. C.) and 450.degree. C. (230.degree. C.) are 12, 7.5, 60 and 300 centipoises respectively. This sharp increase in viscosity causes serious handling problems.
Another problem in dealing with sulfur is that it is a poor conductor of thermal energy, particularly in the liquid state. Furthermore, the thermal conductivity of the sulfur in the liquid state varies with temperature. Consequently, when a solid heating element is placed in contact with a block of sulfur, the liquid sulfur initially formed has the effect of insulating the underlying sulfur from the thermal energy of the heating element. Increasing the temperature of the heating element to drive heat across the insulating liquid boundary layer into the solid sulfur only tends to aggravate the situation, because, as the molten sulfur boundary layer rises in temperature, it undergoes the thickening phase transition stage mentioned above. This thickened sulfur coats the heating element thereby causing a further heat barrier which does not readily flow away. Conventional sulfur remelters, because of these problems, tend to be inefficient and have a high energy expenditure for the amount of sulfur remelted.
Finally, since the conventional sulfur remelters are inefficient and expensive to operate, the sulfur is not superheated to any appreciable extent. The liquid sulfur thus has little surplus heat capacity and is therefore prone to freeze quickly in cold or rainy weather which aggravates handling problems. Frozen sulfur lines must be thawed before they can be used again.